Kees Nuyt wrote:
On Mon, 27 Apr 2009 18:25:42 -0700, Richard Elling
<richard.ell...@gmail.com> wrote:
The concern with large drives is unrecoverable reads during resilvering.
One contributor to this is superparamagnetic decay, where the bits are
lost over time as the medium tries to revert to a more steady state.
To some extent, periodic scrubs will help repair these while the disks
are otherwise still good. At least one study found that this can occur
even when scrubs are done, so there is an open research opportunity
to determine the risk and recommend scrubbing intervals.
Some high availablility storage systems overcome this decay
by not just reading, but also writing all blocks during a
scrub. In those systems, scrubbing is done semi-continously
in the background, not on user/admin demand.
Yes and there is a very important point here.
There are 2 different sorts of scrubbing: read and rewrite.
ZFS (today) does read scrubbing, which does not reset the decay
process. Some RAID arrays also do rewrite scrubs which does reset
the decay process. The problem with rewrite scrubbing is that you
really want to be sure the data is correct before you rewrite. Neither
is completely foolproof, so it is still a good idea to have backups :-)
-- richard
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